1. Field of the Invention
The present invention relates to a process for deter-mining the stability of asphaltenes in live oil.
2. Description of the Background
Asphaltenes, substances defined on the basis of their insolubility in paraffinic solvents (for example, n-pentane or n-heptane) are present in almost all crude oils. Asphaltenes have a high aromaticity and molecular weight; they also have a greater content of heteroatoms with respect to the corresponding soluble fraction (malthenes).
The structure and chemical behavior of asphaltenes vary a lot in relation to their origin and preparation process; if particular conditions are reached in the field, they can precipitate, blocking the pores and thus causing serious production problems. They can also precipitate along the production string, increasing the pressure drop and thus reducing the well production.
The problem of well-drilling operators is to know the conditions, particularly temperature and pressure, under which asphaltenes are stable in live oil. The term xe2x80x9clive oilxe2x80x9d refers to crude oil under temperature, pressure, and optionally, dissolved gas conditions, which are encountered in the reservoir, in which the crude oil is present.
In fact, during the extraction of crude oil, it may happen, owing to variations in pressure and temperature, that the asphaltenes present in the oil separate, thus either partially or even totally blocking the crude-oil extraction pipes.
As a result, the well productivity begins to decrease, with evident economic damage.
The present invention relates to a process for determining the stability of asphaltenes in oil fields.
In this process, use is made of the following equations:
(vs/RT) (xcex4axe2x88x92xcex4s)2="khgr"xe2x80x83xe2x80x83(equation 1);
vs=voxo+vgsxgs+vpxpxe2x80x83xe2x80x83(equation 2);
xcex4s=xcex4oxcfx86o+xcex4gsxcfx86gs+xcex4pxcfx86pxe2x80x83xe2x80x83(equation 3);
xcex4a(T)=xcex4a(To) exp[xe2x88x929.1xc2x710xe2x88x924 (Txe2x88x92To)]xe2x80x83xe2x80x83(equation 4).
In the above equations
xcex4s is the solubility parameter of the xe2x80x9csolvent mixturexe2x80x9d,
i.e. of everything but asphaltenes,
xcex4a is the solubility parameter of the asphaltenes,
xcex4o is the solubility parameter of the oil (xcex4sto for stock
tank oil and xcex4lo for live oil),
xcex4p is the solubility parameter of the paraffin used in the titration,
xcex4gs is the solubility parameter of the good solvent optionally added to the oil,
"khgr" is the asphaltene-solvent mixture interaction parameter,
vs is the molar volume of the xe2x80x9csolvent mixturexe2x80x9d,
vo is the molar volume of the oil (vsto for stock tank oil, vlo for live oil),
vp is the molar volume of the paraffin used in the titration,
vgs is the molar volume of the good solvent optionally added to the oil,
R is the universal gas constant,
T is the temperature in Kelvin degrees,
xo is the molar fraction of the stock oil under threshold conditions,
xgs is the molar fraction of the good solvent under threshold conditions,
xp is the molar fraction of the paraffin under threshold conditions,
xcfx86o is the volumetric fraction of the oil under threshold conditions,
xcfx86p is the volumetric fraction of the paraffin under threshold conditions,
xcfx86gs is the volumetric fraction of the good solvent under threshold conditions,
To is the reference temperature, coinciding, from a practical point of view, with the measurement temperature.
The precipitation threshold is defined as the point in which, during the titration, the separation of the asphaltenes begins. The term xe2x80x9ctitrationxe2x80x9d means addition to the oil, to which a good solvent has optionally been added, of an asphaltene precipitant n-paraffin (for example n-heptane), until a concentration is reached which induces the precipitation of the asphaltenes.
The term xe2x80x9cgood solventxe2x80x9d refers to any solvent capable of solvating the asphaltenes, for example aromatic solvents such as toluene; the various titrations are effected by initially adding different quantities of a specific good solvent to the oil.
The term xe2x80x9cstock tank oilxe2x80x9d means oil under surface conditions (pressure of about 1 atmosphere, temperature of about 25xc2x0 C., absence of dissolved gases).
The present invention relates to a process for determining the stability of asphaltenes in oil fields, characterized in that it comprises the following steps:
a) titrations are carried out of stock tank oil, diluted with good solvents, with C5-C20 aliphatic hydrocarbons, preferably C5-C10 paraffins, even more preferably with n-heptane, thus determining the precipitation threshold of the asphaltenes, said threshold being determined at a temperature ranging from 20xc2x0 C. to the field temperature, preferably the field temperature; subsequently by means of equations (1), with "khgr"=0,5, (2) and (3) (referring stock tank oil), xcex4sto (T) and xcex4a (T), are obtained;
b) from the physico-chemical analyses of the stock tank oil and xcex4sto (T) determined in step (a), the boiling point of the residue (Tbp of the residue) is obtained, by means of an equation of state, preferably the RKS equation (Redlich, Kwong, Soave);
c) from the Tbp of the residue determined in (b) and from physico-chemical analyses of the live oil, the experimental data relating to the phase behavior of the live oil are interpolated, in order to improve the representation of the live oil by means of the above equation of state;
d) using the equation of state with the parameters determined in step (c), the vlo and xcex4lo values of the live oil are determined under different T and P conditions of interest;
e) the "khgr" parameter is evaluated for every condition using:
(i) xcex4a referring to the temperature T, this value being equal to that determined in step (a) if the measurement has been effected at the temperature T or, if the measurement in (a) has been effected at a different temperature, calculated from that obtained in (a) by means of equation (4);
(ii) vlo and xcex4lo obtained in (d), these parameters being used in equation (1);
f) the stability of the asphaltenes being correlated to the "khgr" parameter, the asphaltenes are stable when "khgr" less than 0.5, and unstable when "khgr"xe2x89xa70.5.
The precipitation of the asphaltenes can be interpreted as a precipitation of a sterically stabilized colloid; the precipitation start condition (theta point) is expressed by equation (5):
"khgr"="khgr"xcex8xe2x80x83xe2x80x83(equation 5);
wherein "khgr"=the asphaltene-solvent mixture interaction parameter; "khgr"xcex8=0.5.
On applying the Bragg-Williams expression to the above equation, expression (6) is reached:
(vs/RT) (xcex4axe2x88x92xcex4s)2=0.5xe2x80x83xe2x80x83(equation 6).
Equation (6) does not have any dependence on the concentration of the starting solution, but makes the precipitation threshold of the asphaltenes depend only on the interaction parameterxe2x80x94i.e. on the difference of the solubility parameters (xcex4axe2x88x92xcex4s) and on the molar volume of the solvent mixture vs.
This independence of the threshold from the concentration is experimentally confirmed by all determinations described in literature (see for example Hotier G., Robin M. (1983), Revue de l""IFP, 3-8 (1):101); a strong indication of the model capacity is therefore to reproduce the physics of the phenomenon.
Equation (6) was applied to the process of the present invention.
The first step (a) of the present invention consists in a series of titrations of a stock tank oil diluted with good solvents, preferably aromatic, for example toluene, with various C5-C20 hydrocarbons, preferably C5-C10 paraffins, even more preferably n-heptane. The titrations are carried out at a temperature ranging from 20xc2x0 C. to the field temperature, preferably the field temperature. A typical example of this titration is indicated in FIG. 1, which shows the absorbance trend in a titration with paraffin of an initially stable solution. Each step of the curve corresponds to the addition of a certain volume of paraffin; when the threshold is reached, the asphaltenes precipitate, strongly increasing the absorbance of the solution. Consequently, when the step is no longer horizontal, threshold conditions prevail. The above threshold is determined by means of absorbance measurements, preferably at a wavelength of 800 nm, with a spectrophotometer (see for example Italian patent application IT-A-MI99A 002105 of Aug. 10, 1999) or by other analytical techniques, for example electrically (see for example Italian patent application IT-A-MI97A 01703 of Aug. 7, 1997).
From the above titrations, which measure the precipitation threshold of asphaltenes, data are obtained which are conveniently indicated in a graph (FIG. 2); the abscissa indicates the good solvent mass/stock tank oil ratio, wherein the threshold precipitant mass/oil mass ratio is indicated in the ordinate. In accordance with literature, this graph is a straight line.
From the titrations described above, the dead oil, paraffin and good solvent masses can be determined, under threshold conditions. If the molecular weights and density of these three components are known, the molar fractions and volumetric fractions appearing in equations (2) and (3) can be easily calculated; whereas for the paraffin and good solvent, the data are available in literature, the molecular weight and density data for the oil are available from routine analysis.
On inserting therefore the above data in equations (2), (3), (4), (6), xcex4sto (T), xcex4a (T) are obtained.
In step (b) of the process of the present invention, the boiling point of the residue (residue Tbp) is obtained. This is made possible by using a state equation in which the physico-chemical data of the stock tank oil xcex4sto (T) obtained in step (a), are inserted.
As is known to experts in the field, there are various state equations, among which the most widely used in the oil industry are the RKS (Redich-Kwong-Soave), PR (Peng-binson) and BWR (Benedict-Webb-Rubin) equations; in the case of the hydrocarbon system of interest,) the RKS equation was used.
Using the above state equation and with the help of a calculated program prepared ad hoc, it is possible to obtain Tbp of the residue, a parameter which cannot be determined experimentally.
The residue Tbp is calculated by assuming that the xcex4sto (T) calculated with the RKS equation is equal to the value obtained in step (a).
The third step (c) of the process of the present invention consists in adapting some of the parameters (binary interaction constants) of the state equation in order to reproduce the phase behaviour of the live oil (bubble pressure and field density). This is possible using the experimental data normally obtained in the so-called xe2x80x9cPVT reportxe2x80x9d which is always prepared for each crude oil.
To obtain the best representation of the live oil by means of the RKS equation, a specific calculation program is used.
Step (d) of the process of the present invention consists in calculating vlo and xcex4lo i.e. the molar volume and solubility parameter of the live oil, using for this purpose the state equation with the parameters adapted as described in points (b) and (c).
The last step (e) of the process of the present invention consists in verifying the stability of the asphaltenes in the well in relation to the temperature and pressure. This is possible by inserting the xcex4a (T) parameter in equation (1).
The xcex4lo and vlo values (lo=live oil) determined in step (d) are inserted in equation (1). An expression is thus obtained whereby it is possible to determine the stability of the asphaltenes in relation to the pressure and temperature.
On effecting this procedure for different temperatures and pressures, it is possible to obtain the phase envelope, corresponding to all the live conditions which make asphaltenes unstable.